Slurry and method for chemical mechanical polishing of copper

ABSTRACT

A copper polish slurry, useful in the manufacture of integrated circuits generally, and for chemical mechanical polishing of copper and copper diffusion barriers particularly, may be formed by combining a chelating, organic acid buffer system such as citric acid and potassium citrate; and an abrasive, such as for example colloidal silica. Alternative copper polish slurries, in accordance with the present invention, may be formed by further combining an oxidizer, such as hydrogen peroxide, and/or a corrosion inhibitor such as benzotriazole. Advantageous properties of slurries in accordance with the present invention include the enhancement of Cu removal rates to &gt;3000 angstroms per minute. This high polish rate is achieved while maintaining local pH stability and substantially reducing global and local corrosion as compared to prior art copper polish slurries. Local pH stability provides for reduced within-wafer non-uniformity and reduced corrosion defects. Furthermore, copper diffusion barriers such as tantalum or tantalum nitride may also be polished with such slurries wherein the oxidizer is not included.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to the field of chemicalmechanical polishing (CMP), and more specifically, to methods andchemistries for providing increased metal polish rates.

[0003] 2. Background

[0004] Advances in semiconductor manufacturing technology have led tothe development of integrated circuits having multiple levels ofinterconnect. In such an integrated circuit, patterned conductivematerial on one interconnect level is electrically insulated frompatterned conductive material on another interconnect level by films ofmaterial such as, for example, silicon dioxide. These conductivematerials are typically a metal or metal alloy. Connections between theconductive material at the various interconnect levels are made byforming openings in the insulating layers and providing an electricallyconductive structure such that the patterned conductive material fromdifferent interconnect levels are brought into electrical contact witheach other. These electrically conductive structures are often referredto as contacts or vias.

[0005] Other advances in semiconductor manufacturing technology havelead to the integration of millions of transistors, each capable ofswitching at high speed. A consequence of incorporating so many fastswitching transistors into an integrated circuit is an increase in powerconsumption during operation. One technique for increasing speed whilereducing power consumption is to replace the traditional aluminum andaluminum alloy interconnects found on integrated circuits with a metalsuch as copper, which offers lower electrical resistance. Those skilledin the electrical arts will appreciate that by reducing resistance,electrical signals may propagate more quickly through the interconnectpathways on an integrated circuit. Furthermore, because the resistanceof copper is significantly less than that of aluminum, thecross-sectional area of a copper interconnect line, as compared to analuminum interconnect line, may be made smaller without incurringincreased signal propagation delays based on the resistance of theinterconnect. Additionally, because the capacitance between twoelectrical nodes is a function of the overlap area between those nodes,using a smaller copper interconnect line results in a decrease inparasitic capacitance. In this way, replacing aluminum basedinterconnects with copper based interconnects provides, depending on thedimensions chosen, reduced resistance, reduced capacitance, or both.

[0006] As noted above, copper has electrical advantages, such as lowerresistance per cross-sectional area, the ability to provide for reducedparasitic capacitance, and greater immunity to electromigration. For allthese reasons, manufacturers of integrated circuits find it desirable toinclude copper in their products.

[0007] While advantageous electrically, copper is difficult to integrateinto the process of making integrated circuits. As is known in thisfield, copper can adversely affect the performance of metal oxidesemiconductor (MOS) field effect transistors (FETs) if the copper isallowed to migrate, or diffuse, into the transistor areas of anintegrated circuit. Therefore copper diffusion barriers must be used toisolate copper metal from those transistor areas. Additionally, unlikealuminum based metal interconnect systems which are formed bysubtractive etch processes, copper interconnects are typically formed bydamascene metal processes. Such processes are also sometimes referred toas inlaid metal processes. In a damascene process, trenches are formedin a first layer, and a metal layer is formed over the first layerincluding the trenches. Excess metal is then polished off, leavingindividual interconnect lines in the trenches. The removal of excesscopper is typically accomplished by chemical mechanical polishing.Although there are many known variations of the damascene method ofmetallization, the most common method for removing the excess copper isby CMP.

[0008] Accordingly, there is a need for CMP methods, materials, andapparatus to polish conductive materials such as copper.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a schematic cross-sectional view of a copper damascenestructure. This structure represents a post-plating, pre-polishing stateof fabrication.

[0010]FIG. 2 is a flowchart showing the operations in a process offorming a slurry in accordance with the present invention

[0011]FIG. 3 is a flowchart showing the operations in a process ofpolishing a thin film in accordance with the present invention.

[0012]FIG. 4 is a flowchart showing the operations in a process ofpolishing a thin film in accordance with the present invention.

DETAILED DESCRIPTION

[0013] Methods and slurries for the chemical-mechanical polishing ofcopper are described. In the following description numerous specificdetails are set forth to provide an understanding of the presentinvention. It will be apparent, however, to those skilled in the art andhaving the benefit of this disclosure, that the present invention may bepracticed with apparatus and processes that vary from those of theillustrative examples provided herein.

[0014] Terminology

[0015] The terms, chip, integrated circuit, monolithic device,semiconductor device or component, microelectronic device or component,and similar terms and expressions, are often used interchangeably inthis field. The present invention is applicable to all the above as theyare generally understood in the field.

[0016] RPM (also rpm) refers to revolutions per minute.

[0017] Reference herein to “one embodiment”, “an embodiment”, or similarformulations, means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment of the present invention. Thus, theappearances of such phrases or formulations herein are not necessarilyall referring to the same embodiment. Furthermore, various particularfeatures, structures, or characteristics may be combined in any suitablemanner in one or more embodiments.

[0018] Overview

[0019] Polishing of copper metal layers in connection with the formationof conductive interconnect lines for integrated circuits is becomingmore important for the semiconductor industry. Unlike aluminummetallization, which is typically formed on integrated circuits bysubtractive metal etch, copper interconnect lines are typically formedby way of a damascene, or inlaid, metal process. Such a process requiresthe removal, typically by chemical mechanical polishing, of the excesscopper.

[0020] Several prior art slurries for chemical mechanical polishing ofcopper have had problems associated with them. For example, one suchprior art slurry, based on a hard abrasive such as Al₂O₃, tended tocause excessive scratching and had an unpleasant odor. In another priorart example, a copper polish slurry contained propianic acid and asilica abrasive but had unsatisfactory characteristics with respect tocorrosion, scratching, and odor.

[0021] An exemplary copper polish slurry, in accordance with the presentinvention, may be formed by combining a chelating, organic acid buffersystem such as citric acid and potassium citrate; and an abrasive, suchas for example colloidal silica and an oxidizer, such as hydrogenperoxide (H₂O₂) Alternative copper polish slurries, in accordance withthe present invention, may be formed by further combining a corrosioninhibitor such as benzotriazole (BTA).

[0022] Advantageous properties of slurries in accordance with thepresent invention include the enhancement of Cu removal rates to >3000angstroms per minute. Additionally, this high polish rate is achievedwhile maintaining local pH stability and substantially reducing globaland local corrosion as compared to prior art copper polish slurries.Those skilled in the art will appreciate that local pH stabilityprovides for reduced within-wafer non-uniformity and reduced corrosiondefects.

[0023] The Slurry

[0024] Slurries, in accordance with the present invention, include abuffer system to increase the polish rate of a metal CMP system. Theseslurries are formed by combining a chelating organic acid buffer systemsuch as citric acid and potassium citrate, with an abrasive such ascolloidal silica. If the metal to be polished is copper or a copperalloy, then an oxidizer such as hydrogen peroxide should be combinedwith the slurry mixture. It will be appreciated by those skilled in theart that combining such ingredients may be done in any appropriatecontainer, and may include mixing. Furthermore these ingredients may becombined outside of a container, such as, for example on a polishingpad. Alternative inventive slurries may be formed by further combiningthe above with a corrosion inhibitor such as benzotriazole. Suchslurries are particularly useful for polishing copper, and copperdiffusion barriers.

[0025] An exemplary slurry, in accordance with the present invention,for chemical mechanical polishing, has a pH of approximately 3.8, andincludes a SiO₂ abrasive, a H₂O₂ oxidizer, a benzotriazole corrosioninhibitor, and a citric acid/potassium citrate buffer system. Theseingredients are combined, typically with water, to form the slurry.Those skilled in the art will appreciate that the slurry is a mixture ofthese ingredients, that various chemical reactions may occur amongst theingredients, and that the slurry may contain various mixture andreaction products of the ingredients, including, but not limited to,complexes and disassociated ionic species. In other words, the slurrythat results from combining, or mixing the ingredients, will contain atequilibrium, or at such other conditions as it may be subjected to,chemical constituents that arise by virtue of the combination of theingredients in accordance with the present invention. It is noted thatslurries in accordance with the present invention may have a pH in therange of 3 to 6.

[0026] In one particular illustrative slurry, the citric acid/potassiumcitrate buffer system is provided by including in the slurry mixtureapproximately 3 g/l of citric acid and approximately 3 g/l of potassiumcitrate.

[0027] An abrasive suitable for use in the embodiments of the presentinvention is a precipitated SiO₂. Precipitated SiO₂ is sometimesreferred to in this industry as colloidal, although this term, i.e.,colloidal, is not a technically accurate designation for this material.The illustrative slurry may contain 5 wt. % silica such as Klebesol1498-50 (available from Rodel, Inc., 3804 East Watkins Street, Phoenix,Ariz. 85034).

[0028] The illustrative slurry may further be formed from combininghydrogen peroxide with the slurry mixture such that this oxidizercomprises 3 wt. %. Benzotriazole may be combined with the slurry mixtureas the corrosion inhibitor. In the illustrative embodiment, the slurrymixture includes 0.015M benzotriazole.

[0029] Method

[0030] In an embodiment of the present invention, a copper damascenestructure is polished to form individual interconnects. FIG. 1 shows acopper damascene structure prior to the removal of the excess copper andcopper diffusion barrier layer. An interlayer dielectric (ILD) layer ispatterned to form ILD 102 on a surface of a wafer as illustrated in thefigure. ILD 102 has a thickness represented by TILD in FIG. 1. A copperdiffusion barrier 104 is formed over the exposed surfaces of the waferand ILD 102. Various materials may be used as the copper diffusionbarrier. Tantalum and tantalum-nitride may each be used as copperdiffusion barriers. Typically, a copper seed layer is then formed oncopper diffusion barrier 104. A complete copper layer 106 is thenformed, typically by plating, over diffusion barrier 104. That portionof the copper that is above the top surface 103 of ILD 102 is consideredto be excess. It can be seen by inspection of FIG. 1 that removal of theexcess copper will result in the formation of two separate conductiveinterconnect structures.

[0031] An embodiment of the process of forming a slurry in accordancewith the present invention is illustrated in the flow diagram of FIG. 2.

[0032] As shown in block 202 of FIG. 2, a chelating organic acid buffersystem and an abrasive are combined with water. In one embodiment thechelating organic acid buffer system is citric acid and potassiumcitrate, and the abrasive is colloidal silica. In block 204 an oxidizeris combined with the previously described mixture. In one embodiment theoxidizer is a low electrochemical potential oxidizer such as hydrogenperoxide. In block 206 a corrosion inhibitor is combined with the otheringredients identified above. It will be understood by those skilled inthe art that specific order of introducing the ingredients to the slurrymixture may be changed consistent with the present invention. Thepresent invention is not limited in terms of the order of combiningingredients. For example, water and benzotriazole may be combined, thenthe chelating buffer added, followed by an abrasive, and an oxidizer.

[0033] An embodiment of the method of polishing a thin film on a wafer,in accordance with the present invention, is described in conjunctionwith FIG. 3.

[0034] As is well known, in a typical CMP system, a wafer is placed facedown on a rotating table covered with a polishing pad, which has beencoated with a slurry. A carrier, which may be attached to a rotatableshaft, is used to apply a downward force against the backside of thewafer. A retaining ring may be used to center the wafer onto the carrierand to prevent the wafer from slipping laterally. By applying thedownward force, and rotating the wafer, while simultaneously rotating apad having slurry thereon, a desired amount of material may be removedfrom the surface of a thin film.

[0035]FIG. 3 shows a flow diagram of a process embodying the presentinvention. At block 302, a slurry, having a chelating organic acidbuffer system in accordance with the present invention, is prepared,delivered to, and dispensed onto, a polishing pad. The slurry, asdescribed above, may have a pH of approximately 3.8. Then, as shown atblock 304, a wafer with a copper damascene structure formed thereon, isbrought into contact with the polishing pad. As shown at block 306 thecopper damascene structure is polished. Typical polishing conditionsusing an orbital polisher (e.g., IPEC 576 Orbital Polisher fromSpeed-Fam IPEC, 305 North 54^(th) Street, Chandler, Ariz. 85226) are adown force of approximately 3.75 psi, a spindle speed of approximately310 rpm, a wafer rotational speed of approximately 19 rpm, a slurry flowrate of approximately 130 ccm, and a delta P of 0.0 psi. Delta P is thepressure difference exerted on the top and bottom of the wafer andallows fine control of the rate at the edge of the wafer. Stackedpolishing pads such as the IC1000, with a Suba-4 sub-pad, both made byRodel, Inc. of 3804 East Watkins Street, Phoenix, Ariz. 85034, may beused with the slurry to polish copper films. Other commerciallyavailable polishing pads may be used with the present invention, forexample FX-9 pads available from Freudenberg of Lowell, Mass.

[0036] Copper diffusion barriers, such as, for example, tantalum ortantalum nitride, are also successfully polished with slurries andpolishing conditions in accordance with the present invention. Inparticular, by leaving out the oxidizer-but including the chelatingorganic acid buffer system tantalum based copper diffusion barriers canbe effectively polished.

[0037] A method of forming copper interconnect in accordance with thepresent invention is described in conjunction with FIG. 4. Referring toFIG. 4, a illustrative method includes forming a copper diffusionbarrier layer over a patterned ILD layer (402). This ILD layer,patterned so as to have trenches and vias therein, may be produced withany of the conventional methods of forming an ILD for damascene metalprocessing. ILD layers may include any suitable dielectric material,including but not limited to, silicon oxide, fluorinedope silicon oxide,carbon-doped silicon oxide, and ILD layers based on materials other thanoxides of silicon, such as, but not limited to organic polymers andporous inorganic materials. In the illustrative embodiment of thepresent invention a tantalum-based copper diffusion barrier is used.Such a barrier layer may be made of tantalum or tantalum nitride. Acopper seed layer is then formed over the copper diffusion barrier layer(404). Subsequently, a copper layer is electroplated over the seed layer(406). The excess portion of the copper layer (as described above withreference to FIG. 1) is then removed by chemical mechanical polishing(408) with a slurry that includes a chelating organic acid buffer systemand a low electrochemical potential oxidizer. Such a slurry may containa citric acid/potassium citrate chelating organic acid buffer system,along with hydrogen peroxide as the oxidizer. An abrasive such as silicais also included in the slurry. A corrosion inhibitor such asbenzotriazole may also be included in the slurry. As the copper layer isremoved the underlying diffusion barrier layer becomes exposed. Theexcess portion of the barrier layer, i.e., that portion over the topsurface of the ILD, is then removed (410). The slurry chemistry ismodified such that the oxidizer is left out for removing the excessportion of the diffusion barrier layer. In other words, a first slurryformulation is used when beginning to polish the copper layer, but asecond slurry formulation, similar to the first except for the presenceof the oxidizer, is then dispensed to polish the underlyingtantalum-based diffusion barrier layer.

[0038] With respect to the illustrative embodiment of FIG. 4, copperpolishing and barrier layer polishing may be performed on the same pador on different pads. In the either scenario, copper is polished until apredetermined end point is reached, either by timing the polish, bydetecting a change in CMP motor current, or by any other suitablemethod. If both layers are to be polished on the same pad, the slurrychemistry is modified either by dispensing a second slurry without theoxidizer, or by simply turning off the oxidizer dispenser if this wasbeing delivered directly to the polishing pad. If each layer is to bepolished on separate pads, then when the desired endpoint is detected,the wafer may be moved to a second pad to which the second slurry isdelivered.

CONCLUSION

[0039] Embodiments of the present invention provide a slurry suitablefor chemical mechanical polishing of metals, such as, for example,copper. Other embodiments of the present invention provide methods forforming conductive interconnect lines in an integrated circuit.

[0040] An advantage of some embodiments of the present invention is thatthe chelating agent enhances the copper removal rate to greater than3000 angstroms per minute while using a low electrochemical potentialoxidizer such as hydrogen peroxide. Compatibility with lowelectrochemical potential oxidizers reduces the driving force forpitting and other forms of localized corrosion. A further advantage ofsome embodiments of the present invention is that in the presence of acitric acid buffer system, the concentration of benzotriazole can besignificantly increased to control the static etch rate (sometimesreferred to global corrosion) without shutting down the polish rate.

[0041] A still further advantage of some embodiments of the presentinvention is that the chelating, organic acid buffer enhances theremoval rate in the presence of a soft abrasive such as colloidal SiO₂.

[0042] A still further advantage of some embodiments of the presentinvention is that the buffer system substantially ensures local pHuniformity which, in turn, decreases within-wafer non-uniformity, andalso reduces local corrosion.

[0043] A still further advantage of some embodiments of the presentinvention is that the chelating agent enhances the removal rate over awide pH range and can be used at high pH with a high pH buffer system.

[0044] A still further advantage of some embodiments of the presentinvention is that the chelating, organic acid buffer system, as opposedto conventional slurries, has no substantial stability, odor, health, ordisposal issues associated therewith.

[0045] A still further advantage of some embodiments of the presentinvention is that the ingredients of the slurry form a cost-effectiveproduct.

[0046] A still further advantage of some embodiments of the presentinvention is that an effective slurry for polishing Ta and TaN (i.e.,copper diffusion barriers), can be formed by combining the chelating,organic acid buffer system with an abrasive and a corrosion inhibitor,i.e. the oxidizer component need not be added to the slurry.

[0047] It will be apparent to those skilled in the art that a number ofvariations or modifications may be made to the illustrative embodimentsdescribed above. For example, various combinations, slurry pH, slurrydelivery rate, pad rotation speed, pad temperature, and so on, may beused within the scope of the present invention.

[0048] Other modifications from the specifically described apparatus,slurry, and process will be apparent to those skilled in the art andhaving the benefit of this disclosure. Accordingly, it is intended thatall such modifications and alterations be considered as within thespirit and scope of the invention as defined by the subjoined claims.

What is claimed is:
 1. A method of forming copper interconnect,comprising: forming a copper diffusion barrier layer in at least adamascene structure; forming a copper layer over the barrier layer;removing a portion of the copper layer by chemical mechanical polishingwith a slurry comprising a chelating organic acid buffer system,colloidal silica, and an oxidizer.
 2. The method of claim 1, wherein theoxidizer comprises hydrogen peroxide.
 3. The method of claim 2, whereinthe chelating organic acid buffer system comprises citric acid andpotassium citrate.
 4. The method of claim 3, wherein the slurry furthercomprises a corrosion inhibitor.
 5. The method of claim 4, wherein thecorrosion inhibitor comprises benzotriazole.
 6. A method of formingcopper interconnect, comprising: forming a barrier layer over asubstrate having at least one trench therein; forming a copper seedlayer on the surface of the barrier layer; forming a copper layer overthe barrier and seed layers; removing a portion of the copper layer bychemical mechanical polishing with a first slurry comprising a chelatingorganic acid buffer system, colloidal silica, and an oxidizer; andremoving at least a portion of the barrier layer by chemical mechanicalpolishing with a second slurry comprising a chelating organic acidbuffer system, and colloidal silica; wherein the second slurry is formedwithout the oxidizer.
 7. The method of claim 6, wherein the barrierlayer comprises tantalum.
 8. The method of claim 7, wherein thechelating organic acid buffer system comprises citric acid and potassiumcitrate.
 9. The method of claim 8, wherein the oxidizer compriseshydrogen peroxide.
 10. The method of claim 9, wherein the first slurryfurther comprises a corrosion inhibitor.
 11. The method of claim 10,wherein the first slurry has a pH in the range of 3 to 6, and thecorrosion inhibitor comprises benzotriazole.
 12. A slurry produced bythe process comprising: combining citric acid, potassium citrate,silica, hydrogen peroxide, and benzotriazole.
 13. The slurry produced bythe process of claim 12, wherein a concentration of citric acid isapproximately 3 g/l, a concentration of potassium citrate isapproximately 3 g/l, a concentration of silica is approximately 5 wt. %,a concentration of hydrogen peroxide is approximately 3 wt. %, and aconcentration of benzotriazole is approximately 0.015 molar.
 14. Theslurry produced by the process of claim 13, further comprising combiningthe citric acid, potassium citrate, silica, hydrogen peroxide, andbenzotriazole with water.
 15. A slurry, comprising: approximately 3grams/liter of citric acid; approximately 3 grams/liter of potassiumcitrate; approximately 5 wt. % silica; approximately 3 wt. % hydrogenperoxide; approximately 0.015 molar benzotriazole; and the mixture andreaction products thereof.
 16. The slurry of claim 15, wherein theslurry has a pH in the range of 3 to
 6. 17. A slurry formed by theprocess of combining a organic acid, an organic acid salt; approximately5 wt. % silica; approximately 3 wt. % hydrogen peroxide; andapproximately 0.015 molar benzotriazole.
 18. The slurry of claim 17,wherein the organic acid comprises acetic acid.
 19. The slurry of claim18, wherein the organic acid salt comprises potassium acetate.
 20. Theslurry of claim 17, wherein the organic acid comprises 3 grams/liter ofcitric acid, and the organic acid salt comprises 3 grams/liter ofpotassium citrate.
 21. A slurry for polishing copper diffusion barriers,comprising: approximately 3 grams/liter of citric acid; approximately 3grams/liter of potassium citrate; approximately 5 wt. % silica;approximately 0.015 molar benzotriazole; and the mixture and reactionproducts thereof.
 22. The slurry of claim 21, wherein the copperdiffusion barriers comprise tantalum.
 23. The slurry of claim 21,wherein the slurry has a pH in the range of 3 to
 6. 24. A slurry forpolishing barriers comprised of tantalum, comprising: organic acid, anorganic acid salt, an abrasive, a corrosion inhibitor, and the mixtureand reaction products thereof, and wherein no oxidizer is included. 25.The slurry of claim 24, wherein the organic acid comprise citric acid.26. The slurry of claim 24, wherein the corrosion inhibitor comprisesbenzotriazole, and wherein the slurry has a pH in the range of 3 to 6.27. The slurry of claim 25, wherein the organic acid salt comprisespotassium citrate.